1. Field of the Disclosure
The present disclosure relates to devices incorporating magnetic sensors and more particularly to systems including inertial gyroscopic sensors.
2. State of the Art
Low cost inertial and other sensors became very popular in numerous consumer electronic devices such as smart phone, game controllers, and many others. This imposes very challenging requirements on the developers. The less expensive are products, the less technically sophisticated its customer can be expected and the product should provide robust and care-free operating experience even while using low cost low precision sensors.
It is a common practice in the device manufacturing, to calibrate all the sensors employed by the device at the time of device manufacturing before it is shipped to the customer. Such, so called factory calibration, can be performed very accurately using special equipment, controlled environment, and trained personnel. However, by the time the product is used by the customer the sensor calibration is off due to the temperature change, different battery voltage, and other effects that are beyond control of the design engineers.
This issue of proper calibration is especially important for the sensors, which, in order to be used in the navigation or attitude determination solution, need to be integrated over time. With integration calibration errors grow with time and make the resulted solution very quickly unacceptable.
This means that to ensure proper device operation, for every sensor used in the device, there should be an appropriate sw and algorithms that allow for autonomous real time calibration of such effects as signal drift, and effective gain change which happen during normal device operation due to the environmental effects listed above. In other words, a practical design must incorporate internal algorithms that autonomously monitor and adjust sensor calibration parameters in real time.
This also means that there should be methods that allow reducing the effect of integrated sensor error.
It is, therefore, an object of the present disclosure to resolve one or more of the foregoing difficulties by providing a method that automatically adjusts calibration parameters of the gyro sensors in real time during normal device operation.
It is also an object of this disclosure to provide a method that allows reduction of error in gyro integration.
A typical system today has a full set of sensors such as 3-axis accelerometer, 3-axis magnetic sensor, and 3-axis gyro sensor. All these sensors need to be calibrated to allow effective use. The most common way of accelerometer calibration is to use time moments when system is stationary and to use the gravity vector as the natural standard to determine accelerometer bias and gain. These stationary situations can be set up by the user during user calibration or detected during real time operation, e.g. via “zero motion detector” as in U.S. Pat. No. 5,991,692 since in a stationary position only gravity vector effects accelerometer readings. Using multiple measurements with device in different orientation, a full accelerometer calibration can be performed. As an example see U.S. Pat. No. 6,729,176.
Magnetic sensors are commonly calibrated using the natural Earth magnetic field. The calibration can be done by recording measurements in several predetermined device orientations, or as a background process using operational or user calibration device 3D motion, e.g., US Patent Application Publication No. 20090070056. Such procedure creates new calibration parameters every time the calibration is performed.
Since GPS sensor became widely used and very common in navigation systems where magnetic sensors are employed, it is often used for the calibration of magnetic sensors by comparing GPS velocity vector direction with the direction obtained from the magnetic sensor. Then direct computation of Kalman Filter procedure is used to derive the required calibration parameters for the magnetic sensors.
There is a significant body of work that allows a person who is of ordinary skills in the art, to derive an appropriate procedure to accurately calibrate magnetic and accelerometer sensors used in the device by using natural and ever present forces of Earth Gravity and Earth Magnetic field which are well known and tabulated with high accuracy at any location on the Earth, and by using such additional sensors as GPS and Temperature.
As anyone skillful in the arts is aware, when device orientation is derived from the measurements of the well calibrated magnetic and gravity sensors, each attitude determination is independent. Therefore, unavoidable errors in determination of the magnetic and gravity vector directions are not accumulated in time. Quite opposite, by smoothing the resulted trajectory the orientation error in each trajectory point can be reduced.
However, in the situations when device experiences an unknown acceleration, the internal accelerometers cannot be used to determine device orientation relevant to the Earth gravity vector. Magnetic vector alone is not sufficient to restore device orientation, nor a single antenna GPS can be used to restore body orientation. In such cases one has no alternative but to employ gyro sensors to determine device 3D orientation.
The Gyro sensors are measuring a rate of turn around its local coordinate axis. When properly integrated, this provides a total 3D body rotation from its initial orientation. However, since gyro signal must be integrated to obtain orientation any error in gyro bias or gain rapidly grows in time.
Gyro bias can be determined by observing gyro signals at the moment when there is no rotation is present, assuming that such moments can be detected. However, to calibrate gyro gain one needs to perform very accurate and well known rotation around each axis which is difficult to perform even in factory setting and all but impossible for the in-field real time calibration.
Tekawy, et al., U.S. Pat. No. 7,667,645, U.S. Pat. No. 7,393,422 is using power difference from different GPS satellites in multiple GPS antennas to calibrate gyro drift.
Drag, et al., U.S. Pat. No. 7,657,183, uses optical sensors as a primary method of orientation. This patent mentions that gyro can be calibrated using these optical signals but does not explain how it can be done.
Achkar, et al., U.S. Pat. No. 5,562,266, is also using Sun and North Pole detectors to estimate gyro drift (bias) via a Kalman Filter, which is also similar to the method used by Basuthakur, et al, U.S. Pat. No. 5,452,869
Therefore, at the present, the gain calibration methods need to rely on the external determination of the device trajectory orientation, usually GPS for vehicle, star finder for satellites, or sonar for underwater apparatus. However, such methods don't work when device experiences rotation which is independent on its trajectory which is a common case for the hand held devices.
It is an object of the present disclosure to provide a practical and efficient method that automatically adjusts calibration parameters of the gyro sensors in real time during normal device operation. It is also an object of this disclosure to provide a practical and efficient method of improving accuracy of the device orientation determination.